Monitoring cameras are used in many different applications, both indoors and outdoors, for monitoring a variety of environments. Digital monitoring cameras use image sensors that are sensitive to a wider spectral range of light than the human eye. Digital image sensors, such as CCD sensors and CMOS sensors, are sensitive not only to visual light, but also to part of the IR spectral range. In a well-lit scene, the IR component is generally a disadvantage, as it distorts the colours of captured images compared to what the human eye can see. Therefore, an IR cut filter is normally placed in front of the image sensor in a day-mode of the camera, such that only visual light reaches the image sensor. When there is little visual light in the scene, such as at night, the sensitivity to IR light may be used to advantage. In a night-mode of the camera, the IR cut filter is removed, such that also IR light may reach the image sensor. In the night-mode, any colour information available is disregarded, and only light intensity is used, such that a black and white image is produced.
IR light is in most cases available to some extent in the scene, such that images may be captured also at night. Still, in many cases there is a need to add light. IR illuminators may therefore be arranged in the scene. These may be separate units, arranged some distance from the camera, but may be controlled by or in conjunction with the camera, such that the IR illuminator is turned on when the camera determines that there is too little light in the scene. Some IR illuminators are mounted on the outside of the camera, such as a half-circular illuminator marketed by Axis Communications AB under the name AXIS T90C10 Fixed Dome IR-LED.
Cameras may also be equipped with integrated IR illuminators, arranged inside a dome of the camera. A problem with such an arrangement is that there is a risk that light from the illuminator is reflected on the inside of the dome and enters the camera, thereby causing complete or partial saturation of the image sensor. To prevent this risk, a gasket is placed around the camera for sealing against the dome. If the dome is not mounted carefully, stray light from the illuminator may still be reflected and reach the image sensor. This becomes even more problematic in camera arrangements having more than one camera head, as each camera head has to be sealed off properly.
Some cameras do not have a dome, such that an IR illuminator may be integrated in the camera without the risk of internal reflections. However, this solution comes at the cost of a less protected camera, making the camera unsuited for applications where there is a risk of, e.g., vandalism.
So far, IR illuminators have been discussed, but it should be noted that there are also illuminators emitting light in the visible spectral range. The problems described above in connection with IR illuminators apply also to visible light illuminators. The choice of which spectral range to illuminate the scene with depends on the specific monitoring situation. An illuminator providing visible light may have a great deterring effect. For instance, such an illuminator may be activated based on a motion detection event, such that when a person enters the monitored scene, light is turned on, making the person realise he or she has been detected. Still, there are situations when visible illumination would be annoying, e.g., to neighbours. There are also situations where a more discrete or covert surveillance is desired. In the two latter examples, an IR illuminator may be a better choice, since such illumination will be less noticeable or even invisible. IR illuminators may emit different wavelengths. Some IR illuminators emit wavelengths of, e.g., 730 nm or 850 nm, and these wavelengths generally produce a faint glow in the vicinity of the illuminator. Other IR illuminators operate at 940 nm, which is completely outside the visible spectral range and may be used for covert surveillance.
Regardless of the spectral range emitted by the illuminator, there is still a need for improvements in the arrangement of illuminators in connection with cameras.